The field of the invention pertains to spectrophotometry and, in particular, to rapid-scan spectrophotometers and the speed with which such instruments can scan the relevant spectra.
In well known mechanically scanning spectrophotometers the entrance and exit slits are located on either side of the optical grating. A simple elliptically concave mirror is used as a collimating and focusing mirror intersecting and directing the light from an ultraviolet or visual light generator for a UV/VIS spectrophotometer. The light beams entering the monochromator strike the left side of the mirror, are collimated and reflected to the grating. The diffracted radiation goes to the right half of the same mirror and is focused on the exit slit. The wavelength is selected by simple pivoting of the grating about the monochromator axis. The angle between the incident and diffracted rays remains constant. Either a manual or motor driven sine bar drive produces a direct wavelength readout on a linear scale.
Since the useful range in UV/VIS spectroscopy lies typically within 15 degrees about the grating optical axis, the grating is rotated back and forth over this 15 degree range to scan the region of interest. Mechanical scanning of the desired spectrum is achieved through a device such as a stepping motor. The information from a shaft encoder thereattached is used to translate the angular position of the grating into a wavelength.
Spectrophotometers that rely upon such electromechanically reversing arrangements for the grating cannot truly be considered rapid scanning, because they typically scan about 400 to 2400 nm per minute. The arrangement cannot be increased in speed because of the mechanical cam shaft follower drive and the need to determine the grating position accurately.
Full electronic spectroscopy has been achieved with diode array spectrophotometers that scan the range of 200 to 800 nm 5 to 10 times per second. Such spectrophotometers require custom-made circuits with attendant high cost for limited production. Diode arrays have a limited spectral response and extension into the near and far infra red remains unavailable without arrays of hundreds or thousands of elements.
High-throughput spectroscopy can also be accomplished with a fast mechanical scanner with all reflective optics. Scanning is achieved by vibrating a low-inertia grating or mirror as disclosed in U.S. Pat. No. 4,225,233 and the paper by J. Stoijek and Z. Uziel, Pol. J. Chem., 53, 1619 (1979). The mirror or grating (depending on the optical configuration) is mounted directly on the output shaft of a galvanometer type optical scanner, where the position is a function of the applied electric current. By changing the source, grating, and detector, a wide wavelength range can be covered. A commercial device based on U.S. Pat. No. 4,070,111 is available presently with a vibrating grating. Unfortunately, the scanning speed, although much greater than with the electro-mechanical scanner above, causes increased optical difficulties. To minimize inertia the grating or mirror is very small. A large number of optical elements, fixed magnification between the entrance and exit slits and a high energy input light source are required.
U.S. Pat. No. 4,245,911 discloses a drum cam mechanical drive to oscillate the grating and means to adjust the scanning speed. U.S. Pat. Nos. 4,264,205 and 4,285,596 disclose a conjugate cam mechanical drive to oscillate the grating. In both disclosures the mechanical drive is directed to retaining the accuracy of the mechanical drive and to eliminate play in the mechanical parts thereby reducing noise in the measurements at higher scanning speeds. All such mechanical oscillating drives for the grating are inertia limited because of the reversal of movement in each cycle.